#include <bits/stdc++.h>
using namespace std;
int n,a[100005];
int main()
{
    freopen("duel.in","r",stdin);
    freopen("duel.out","w",stdout);
    cin >> n;
    for (int i=1;i<=n;i++)
    {
        cin >> a[i];
    }
    cout << n/2-1;
    fclose(stdin);
    fclose(stdout);
    return 0;
}
